usually nga sa tire yan... although the design of the car itself contributes a lot too. yung ibang cars, like the EK civic, talagang pumapasok yung road noise as if kakaunti yung insulation (from experience)...

may ibang surfaces naman na talagang maingay... usually ito yung coarse na asphalt. you'll hear the droning noise in any car, pero syempre, more in others.